Nuclear spin dependence of the reaction of H3+ with H2. II. Experimental measurements

The nuclear spin dependence of the chemical reaction H-3(+) + H-2 -> H-2 + H-3(+) has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho-and para-H-3(+) (o-H-3(+) and p-H-3(+)) in hydrogenic plasmas of varying para-H-2 (p-H-2) enrichment. The ratio of the rates of the proton hop (k(H)) and hydrogen exchange (k(E)) reactions alpha equivalent to k(H)/k(E) is inferred from the observed p-H-3(+) fraction as a function of p-H-2 fraction using steady-state chemical models. Measurements have been performed both in uncooled (T-kin similar to 350 K) and in liquid-nitrogen-cooled (T-kin similar to 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of a has been found to decrease from 1.6 +/- 0.1 at 350 K to 0.5 +/- 0.1 at 135 K. (C) 2011 American Institute of Physics. [doi:10.1063/1.3587246]